Many of his colleagues throughout the league joined the protest, which hit TV ratings and allegedly contributed to Kaepernick being able to find a team for this season.

And addressing a rally in Alabama last night, Trump said: "Wouldn't you love to see one of these NFL owners, when somebody disrespects our flag, to say 'Get that son of a b***h off the field right now! He is fired. He's fired!'

"When people like yourselves turn on television and you see those people taking the knee when they are playing our great National Anthem.

"The only thing you could do better is, if you see it – even if it's one player – leave the stadium. I guarantee things will stop. Things will stop.

"Pick up and leave. Just pick up and leave.

"You know, some owner is going to do that. He is going to say, 'That guy that disrespects our flag, he's fired'.

NFLPA executive director DeMaurice Smith said: “Whether or not Roger and the owners will speak for themselves about their views on player rights and their commitment to player safety remains to be seen.

“This union, however, will never back down when it comes to protecting the constitutional rights of our players as citizens as well as their safety as men who compete in a game that exposes them to great risks."

Washington Redskins star Zach Brown was among the players to hit back at Trump, telling the president to stay in his place.